1. Field of the Invention
This invention relates to a box annealing furnace for annealing metal sheets such as cold-rolled carbon steel sheets, for example. This invention also relates to a method for making metal sheets including strips and coils, in addition to cut sheets, all with the use of a furnace of this invention. The invention further relates to the cold-rolled and annealed products of the method.
2. Description of the Related Art
Cold-rolled stainless steel and heat-resisting steel sheets can be produced by hot rolling, hot annealing and pickling, cold rolling-finish annealing and pickling (cold-rolled annealing and pickling), and subsequent skin-pass rolling. The finish annealing and pickling procedure generally comprises a continuous annealing, pickling or continuous bright annealing.
Although such a continuous line is useful for mass production, it is not always appropriate for small, batch-type production. Also, a continuous line is not appropriate for production of cold-rolled stainless steel and heat-resisting steel sheets using a cold rolling production line for carbon steel or general steel. Instead of finish annealing (cold-roll annealing) and pickling requiring huge facilities, the use of box annealing (also called "bell annealing" or "batch annealing") is economically advantageous in many cases.
A cold-rolled stainless steel and heat-resisting steel sheet or coil, however, is subjected to finishing annealing for a long time in a conventional box annealing procedure in which the oxygen content and dew point in the furnace atmosphere are not decreased sufficiently. Thus, an oxide film having a thickness of 4,000 .ANG. or more may be formed on the steel during finish annealing. The oxide film causes severe defects in the stainless steel and heat-resisting steel sheet, one of which is a surface discoloration called temper discolor. Another serious defect resides in deterioration of corrosion resistance (see FIG. 4 discussed hereinafter). Accordingly, box annealing is not presently used as finish annealing in processes for making cold-rolled stainless steel and heat-resisting steel sheets.
Among cold-rolled steel sheets, temper discolor is also observed in high-manganese steel (manganese content: 0.5 to 1.0 percent by weight), and in high niobium steel (niobium content: 0.2 to 0.5 percent by weight). For example, as shown in FIG. 7, annealing of high manganese steel in a HN (hydrogen 7 percent by volume and nitrogen 93 percent by volume) gas annealing atmosphere under soaking conditions of 680.degree. C. and 30 hours creates a yellowish-brown temper discolor in a region 20 which is approximately 150 to 300 mm distant from the sheet edges. Further, a white temper discolor occurs in region 21, having a width of 50 mm from the sheet edges.
Proposed methods for preventing such temper discolor phenomenon include physical and chemical removal of the oxygen source, for example, improved sealing of the furnace and reduced residual air content in the furnace by evacuation of the gas from the furnace prior to annealing. In addition, Japanese Patent Application Laid-Open No. 54-102222 discloses placement of pure copper in the furnace to remove H.sub.2 O by a reducing reaction. Although the pure copper reliably absorbs oxygen from the furnace atmosphere by oxidation at an initial stage of the annealing process, the resulting copper oxide becomes reduced during a subsequent high-temperature soaking step, and evolves oxygen due to the weak affinity that exists between copper and oxygen. The oxygen gas evolved in the furnace atmosphere causes surface oxidation of the steel during cooling.